Abstract. A continuous Euler model has time‐varying coefficients. Through a logarithmic time transformation, a continuous Euler model can be transformed to a continuous autoregressive (AR) model. By using the continuous Kalman filtering through the Laplace method, this article explores the data application of a continuous Euler process. This time deformation of an Euler process deforms specific time‐variant (non‐stationary) behaviour to time‐invariant (stationary) data on the deformed time scale. With these time‐invariant data on the transformed time scale, one may use traditional tools to conduct parameter estimation and forecasts. The obtained results can then be transformed back to the original time scale. Simulated data and actual data such as bat echolocation and the US residential investment growth are used to demonstrate the usefulness of time deformation in forecasting. The results indicate that fitting a traditional autoregressive moving‐average (ARMA) model on an Euler data set without imposing time transformation leads to forecasts that are out of phase while the forecasts of an Euler model stay mostly in phase. 相似文献
随着计算机技术及感知技术的发展及应用,各个领域普遍出现不确定性数据流形态的新型数据,吸引了众多研究者的关注.现有的数据流聚类技术普遍忽略不确定性特征,常导致聚类结果的不合理甚至不可用.为数不多的针对不确定性特征的聚类方法片面考察不确定性,且大多基于K-Means算法,具有先天缺陷.针对这一问题展开研究,提出了不确定度模型下数据流自适应网格密度聚类算法(adaptive density-based clustering algorithm over uncertain data stream,ADC-UStream).对于不确定性特征,该算法在存在级和属性级不确定性统一策略下,构建熵不确定度模型进行不确定性度量,综合考察不确定性.采用网格-密度的聚类算法,基于衰减窗口模型设计时态和空间的自适应密度阈值,以适应不确定性数据流的时态性和非均匀分布特征.实验结果表明,不确定模型下的数据流网格密度自适应聚类算法ADC-UStream在聚类结果质量和聚类效率方面都具有较好的性能. 相似文献
This paper presents a mathematical model based on the reaction rate expressions to describe the displacement of methane conversion in the steam reforming. The effect of several parameters including weight hourly space velocity (WHSV), load-to-surface ratio, reaction pressure, hydrogen partial pressure in permeate side and reaction temperature were investigated. Simulation and experimental results showed that a conversion higher than 80% could be achieved in a palladium membrane reactor at reaction temperature of 500 °C relative to 850 °C in a conventional fixed bed reactor (FBR). Besides, the yield of CO (<2%) in membrane reactor was much lower than that (>50%) in the FBR, which indicated the significant depression of CO production in use of membrane reactor. 相似文献
Spherulitic growth rates and microstructure of syndiotactic polystyrene (sPS) cold-crystallized isothermally at various temperatures, Tc (115–240 °C), have been investigated by small-angle light scattering (SALS), optical microscopy and transmission electron microscopy. The derived activation energy for sPS chain mobility at the crystal growing front is 5.4 kJ/mol, which is relatively lower than that of isotactic polystyrene, 6.5 kJ/mol. In addition, the Hv scattering invariant (QHv) measured by SALS on the crystallized sPS samples displays a pronounced minimum at 150 °C. Despite a wide range of Tc used, however, the sample crystallinity estimated by Fourier transformation infrared spectroscopy remains unchanged. Prior to crystallization, the correlation length derived from the Vv patterns on the basis of Debye–Bueche model is ca. 1.13 μm regardless of Tc used. Interconnected domains with a width of ca. 1.8±0.5 μm are readily observed in all the crystallized samples under phase contrast microscopy and the phase-separated structure is conserved within sPS spherulites whose diameters are increased with increasing Tc.
Based on the above facts, we conclude that the presence of a QHv minimum is ascribed to the resultant events of the two competitive transitions i.e. liquid–solid crystallization, and liquid–liquid demixing resulting from the spinodal decomposition (SD). At lower Tc, the unstable SD transition overwhelms the crystallization. Despite the low chain mobility, the coarsening process driven by the interfacial energies has reached a certain level before crystalline nucleation takes place. At higher Tc, on the other hand, cold crystallization becomes the dominant process due to the enhanced chain mobility, leading to the suppression of ongoing SD coarsening process. At an intermediate Tc range, comparable competition of the phase separation and crystallization prohibits the development of ordered symmetry within spherulites, giving the presence of QHv minimum. 相似文献